1. Field of the Invention
The present invention relates to the structure of a light-emitting diode (LED) and method of manufacturing the same.
2. Description of Prior Art
The AlGaInP alloy system, which is a semiconductor with a direct energy bandgap, has been used for making high quality semiconductor lasers with an emitting wavelength of around 670 nanometers. This alloy system may also be useful for making light-emitting diodes (LEDs) for wavelengths ranging from about 560 to 680 nanometers by adjusting the aluminum to gallium ratio in the active region of the device. Increasing the aluminum proportion produces shorter wavelengths. However, the AlGaInP substance has a still lattice constant matched with the GaAs substrate while the ratio of aluminum and gallium is varied. It has also been demonstrated that metal organic vapor phase epitaxy (MOVPE) provides a means for growing optically efficient AlGaInP hetero-structure devices.
A general structure adopted for the AlGaInP double hetero-structure LED is: forming a n-type AlGaInP cladding layer on a n-type GaAs substrate; then growing an undoped AlGaInP active layer thereon; and growing a p-type AlGaInP cladding layer on the AlGaInP active layer. The light-emitting efficiency of the AlGaInP LED described above does not depend only on the recombination rate of the electrons and the holes in the active layer, but also depends on the efficiency of current spreading in the upper cladding layer. This is, for efficient operation of the LED, current injected by the front electrical contact should be spread evenly in the lateral direction so that the current will cross the p-n junction of the double hetero-structure of AlGaInP uniformly to thereby generate light evenly. The p-type AiGaInP layer, which is grown by means of the MOVPE process, is very difficult to dope with acceptors of a concentration higher than 1E18 cm.sup.-3. Moreover, it is a material characteristic that hole mobility is low in p-type AlGaInP semiconductors (about 10 to 20 cm.sup.2 .times.V/sec). Due to these two factors, the electrical resistivity of the p-type AlGaInP layer is comparatively high (i.e., about 0.5 .OMEGA.-cm) so that current spreading is severely restricted. As a result, the current tends to concentrate under the front electrical contact. This is often referred to as current crowding.
Some different structures have been developed to solve the above problems in the prior-art LED.
One technique to solve the current crowding problem is disclosed by Fletcher et al in a U.S. Pat. No. 5,008,718 and in the Journal of Electronic Materials, Vol. 20, No. 12, 1991, pp. 1125-1130. The proposed LED structure is shown in FIG. 1, wherein the device geometry of a conventional LED is fabricated with a back electrical contact 110, a substrate of n-type GaAs 120, a double hetero-structure of AlGaInP 130, which includes a layer of n-type AlGaInP 131, a layer of undoped AlGaInP 132, and a layer of p-type AlGaInP 133, and a front electrical contact 150, in which a semiconductor window layer 140 is grown upon the p-type AlGaInP layer 133. The window layer 140 should be selected from materials that have low electrical resistance so that the current can spread out quickly, and a bandgap higher than that of the AlGaInP layers so that the window layer is transparent to light-emitted from the active layer of AlGaInP.
In an LED for generating light in the spectrum from red to yellow, an AlGaAs substance is selected to form the window layer 140. The AlGaAs substance has the advantage of having a lattice constant matched with that of the underlying GaAs substrate 120. In an LED for generating light in the spectrum from yellow to green, a GaAsP or GaP substance is used to form the window layer 140. A drawback of using GaAsP or GaP substances that their lattice constants are not matched with those of the AlGaInP layers 130 and the GaAs substrate 120. This lattice mismatch causes a high dislocation density that produces less than satisfactory optical performance. On the other hand, the window layer has to be grown with a thickness of 5 to several ten micrometers to evenly spread the current. However, this increases the time and complexity for manufacturing an LED due to two growth procedures, i.e. a MOVPE procedure for growing the AlGaInP double hetero-structure and a vapor phase epitaxy (VPE) procedure for growing the thick window layer.
FIG. 2 shows a second prior art LED disclosed in U.S. Pat. No. 5,048,035 by Sugawara et al.. The LED of FIG. 2, in addition to the structure of FIG. 1, is fabricated with a Bragg reflector layer 260, a current-blocking layer 270, a current spreading layer 240, and a ohmic contact layer 245. The current spreading layer 240 has very low electrical resistance and the current-blocking layer 270 is arranged at a position where it is in alignment with the front electrical contact 150 and thus is spread out laterally by the current-blocking layer 270. Moreover, the reflector layer 260 can be used to prevent the light-emitted by the active layers from being absorbed by the GaAs substrate.
A drawback of the LED in FIG. 2 is that the fabricating process, in which the MOVPE procedure needs to be performed twice, is very complex. Moreover, the p-type AlGaInP layer 133 is easily oxidized since it contains a large proportion of aluminum.
Furthermore, light-emitted from the active layer is mostly absorbed by the GaAs substrate since the energy bandgap of the GaAs substrate is less than that of the active layer. To address this drawback in the prior-art LEDs, referring to FIG. 3, another technique is disclosed in U.S. Pat. No. 5,376,580, which includes a n-type GaAs substrate 320, a double hetero-structure of AlGaInP 330, which includes a n-type AlGa InP cladding layer 331, an undoped AlGaInP active layer 332, a p-type AlGaInP cladding layer 333, a p-type GaP window layer 340, a first electrical contact 350 and a second electrical contact 310. This structure is made by etching away the GaAs substrate in an LED as shown in FIG. 1, and bonding a GaP substrate having a thickness of about 350 micrometers to the LED layers having a total thickness of about 50 micrometers utilizing a wafer bonding technique so as to form an AlGaInP LED having a transparent substrate. However, it is very difficult to deal with an LED layer only having a total thickness of about 50 micrometers and to protect the LED membrane from breaking.
The other prior art is disclosed in U.S. Pat. No. 5,317,167. Referring to FIG. 4, the LED includes a p-type AlGaAs holding layer 420, a p-type AlGaInP intermediate layer 430, a double hetero-structure of AiGaInP 440, which includes a p-type AlGaInP cladding 441, an undoped AlGaInP active layer 442 and a n-type AlGaInP cladding layer 443, a front electrical contact 450 and a back electrical contact 410. This prior art grows a AlGaAs layer with a thickness of 30 micrometers by employing MOVPE or LPE (Liquid Phase Epitaxy) after the AiGaInP double hetero-structure has grown. The AlGaAs layer is used as a holding layer of the double hetero-structure. Then, the GaAs substrate is etched away by chemical etching. While manufacturing this prior-art LED, it is difficult to form the 30-micrometer thick membrane, and the transmittance of AlGaAs is worse than that of GaP. Therefore, it is not suitable for being used to fabricate a LED emitting short wavelength (e.g., green light).